Wednesday's iCloud launch preceded the debut of iOS 5 -- which was released around 1 p.m. ET. -- and the upgrade to Mac OS X 10.7, aka Lion to version 10.7.2.

Those upgrades are necessary to access the new service.

Apple unveiled iCloud last June at its annual developers conference, where then-CEO Steve Jobs described the service as the "soul" of iOS, the mobile operating system that powers the iPhone, iPad and iPod Touch.

That presentation was Jobs' last for Apple, and his second-to-last public appearance.

Unlike MobileMe, iCloud is free to all iOS 5 and Lion users. iCloud includes 5GB of free online storage and serves as a music, photo, app, document and other data sync service that keeps multiple devices up-to-date with user-purchased or -created content. Users can purchase additional storage space: $20 annually will cover 10GB of space; $40, for 20GB; and $100 a year, for 50GB.

Users cannot begin accessing iCloud, and MobileMe customers cannot start migrating some, though not all, of the data stored on that three-year-old service until they have iOS 5 installed on a mobile device, or upgrade their Mac to OS X 10.7.2.

Apple has published an FAQ for MobileMe-to-iCloud migrations, warning users that some features of the former -- including iDisk and Gallery -- would not be available in the latter.

Current MobileMe subscribers will be allowed to keep their email address when they shift to iCloud, and can move their email, contacts, calendar and bookmarks to the new service.

MobileMe users have had their subscriptions extended to June 20, 2012, and will be able to access information not migrated -- including the files they've previously stored on iDisk, the service's 20GB online storage locker -- until then.